\section{Acknowledgments} 

We would like to thank Pascal Bourguignon, Stas Boukarev, Dave Fox,
and Hans Hübner for their help with the tests on platforms not at our
disposal.  We would also like to thank Christophe Rhodes for reading
an early draft of the paper and for suggesting improvements, and to
the developers at LispWorks for pointing out the potential influence
of thread-safety on performance.
